Presently, when more than one telephone station has access to the same telephone line and where it is desired that each station have the capability of placing a connection in the hold mode, a line circuit is used to control the telephone connections. Such arrangements are commonly called key telephone systems and the line circuit, or key telephone unit, is used to provide visual and audio indications of the condition of the telephone line as well as to control the hold function.
Typically, a single line circuit is interposed in the telephone line between the central office or PBX switching machine and the station, and arranged to serve as many telephone stations as it is desired to be connected to the same line. However, a problem develops in situations where two telephone stations, separated by large distances, are to share a common line. This problem is manifest in that under such a condition the resistance of the line connecting the two remote stations is such that an excessive voltage drop occurs, preventing the proper operation of the lamp and hold control circuitry.
A further problem is that circuitry added to the key telephone system must be done in a manner which does not require extensive modification to the existing telephone layout. This requirement stems from a desire to avoid the need for custom engineering each system, thereby maintaining the cost at a minimum level. In some prior art attempts to solve the problem, an auxiliary line circuit has been utilized to transmit signals to the far end for control purposes. These circuits, however, have required either extensive circuitry or access to the inner logic of the line circuit. In both of these situations major modifications to the telephone plant are required.
Thus, it is an object of my invention to provide a circuit usable in conjunction with existing circuits for providing lamp control and hold control functions at each of two remotely located stations without regard as to which station initiated the function and without requiring any changes to existing line circuits, to their interconnecting pattern, or to the central switching machine.